Netflix VMAF is an algorithm to generate a video quality score by comparing a reference image/video with a distorted image/video. To do that VMAF calculates scores using tradicional image quality metrics like VIF or DLM and then aggregate them using a Machine Learning model (SVM) trained with the videos and scores coming from real users. Smart, isn't it? (You can see a high level description of those metrics that are aggregated in this Netflix post or the Wikipedia page)

It is important to notice that VMAF works in a per-frame base so it is NOT a good tool to measure the quality impact of many artefacts happening in Real Time Communications (delays, reduced/frozen framerate, audio/video desync). However we can use it to measure the impact of different encoding settings like the average bitrate of the encoding.